1. Field of the Invention
This invention relates generally to measuring instruments and more particularly is directed towards a new and improved head assembly for use with bore gauges of mechanical or electronic construction.
2. Description of The Prior Art
A bore gauge is an instrument designed to measure inside diameters of various cylindrical bores. Machinists, for example, frequently need to make precise measurements of the diameter of a bore being cut into a workpiece. Since the sensing contact of a bore gauge typically has a relatively small range of motion, the effective measurement range of the instrument can be increased by providing an extensible stem directly opposite the sensing contact of the gauge. Such stems heretofore have been in the form of screws that are turned in or out as required with the particular setting being established by inserting the head of the bore gauge in a reference ring selected to correspond approximately with the diameter of the bore to be measured. Once the bore gauge has been set in the reference ring, the head of the instrument is then inserted in the bore itself and a reading is made, typically on a dial indicator mechanically connected to the measuring head.
While this type of instrument is known in the art as exemplified by the following U.S. Pat. Nos. 3,418,720, 3,442,020, 3,422,540, 4,030,202 and 4,170,831, these instruments have been somewhat difficult to use and maintain. For example, the initial setting of the extensible stem is quite slow and may require several adjustments before the stem is correctly extended and locked as by turning a locking nut, which sometimes will cause an accidental movement of the stem. Also, the contacts used both for centralizing the instrument along the diameter of the bore as well as the measuring contacts themselves tend to wear rather rapidly. Likewise, other moving parts tend to experience rather rapid wear resulting in deterioration in the accuracy of the instrument. A further disadvantage of existing bore gauges of this type has been the tendency of parts to become clogged with debris insofar as the instruments are used frequently around machining operations where liquid coolants and the like may be present.
Accordingly, it is a general object of this invention to provide improvements in bore gauges. Another object of this invention is to provide a measuring head for a bore gauge adapted to actuate either a mechanical instrument such as a dial indicator or an electrical instrument such as a linear variable differential transformer coupled to an electrical digital display. A further object of this invention is to provide in a bore gauge a head that may be quickly and easily set in a reference ring prior to use in measuring a bore. Still another object of this invention is to provide in a bore gauge a head that is easily cleaned, is long wearing and requires a minimum amount of maintance while providing very reliable, repeatable and precise measurements.